1. Field of the Invention
The present invention relates to a method and apparatus for producing an intensity contrast from phase detail in a transparent phase object. More particularly, the invention pertains to a method and apparatus for enhancing the "gradient" intensity image through the use of a subtraction process.
2. Description of the Prior Art
Prior methods for producing phase gradient intensity images from transparent phase objects have been fully described in U.S. Pat. No.: 4,721,362 to Brody et al., assigned to the U.S. Government and is hereby incorporated by reference. In this method a beam of collimated coherent light is produced and directed through a transparent phase object. The beam of collimated coherent light, after it has passed through the transparent phase object, is directed to photorefractive holographic means for producing and recording a hologram such that the phase conjugate beam of the transparent phase object is generated from the hologram. The position of the transparent phase object is shifted, and the phase conjugate beam of the transparent phase object, after it has passed through the shifted transparent phase object, is directed to means for observing the intensity image of the component of the spatial derivative of optical phase retardation in a particular direction. The photorefractive holographic means for producing and recording a hologram in this method may comprise a photorefractive hologram recording crystal. This crystal may comprise an oriented crystal of barium titanate in the form of a parallelpiped.
The hologram is produced by passing the input coherent optical field through the specimen and into the crystal. The formation time is dependent on the input optical power. The time is characteristically several seconds at the 200-mW power levels. Once formed, the hologram updates as the input changes. Updating occurs with relative rapidity, with lags of only a fraction of a second between input and formation.
The phase conjugate of the transmitted original optical field is produced when the hologram (within the crystal) is illuminated by an input coherent field from the laser. This phase conjugate field propagates from the crystal and passes back through the specimen. The consequence of the backward passage through the original phase distorting object is the removal of the phase distortions. The input optical field is the coherent output of a laser. The phase fronts are planes, and the intensity distribution is Gaussian. As a result, the back-propagating field after its passage through the original (unshifted) phase-distorting specimen does not contain intensity features other than those found in the original Gaussian input. However, when the subject through which the phase-conjugate field passes is not the original subject, but the original subject shifted transversely, then there will be intensity features in the back-propagating field. These will depend on the magnitude of the shift.
The problem with generating intensity images by this method is that the images include intensity mottling and spurious unchanging background patterns from the coherent light source. These spurious patterns degrade the gradient image formed purely by the shifted specimen.
Another disadvantage to the prior art method is that it is cumbersome if not impossible to use when trying to image naturally moving specimens. The prior art also produces only images related to the gradient of phase retardation in the shift direction. It is sometimes desirable to produce images related more closely to the total phase change due to transmission rather than the aforementioned phase gradient images.